This invention relates to the detection of plastics and other materials using dielectrokinesis (phoresis) and, more particularly, to the detection of specific plastics, polymers and other organic and inorganic materials via detection of an electrodynamic reaction current surge to mechanically-constrained, inverse dielectrophoresis force.
Detection of specific polymers and plastics (blends and mixtures of various polymers and with additives) and other organic/inorganic materials, irrespective of the presence of intervening vision-obstructing structures, barriers or EMI signals has uses in very diverse applications such as (a) transportation security in pre-boarding planes, trains and automobiles; (b) new and old construction; (c) law enforcement; (d) military operations; (e) anti-shoplifting protection; and (f) other security needs and operations.
Dielectrophoresis describes the force upon and mechanical behavior of initially charge-neutral matter that is dielectric polarization charged via induction by external spatially non-uniform electric fields. The severity of the spatial non-uniformity of the electric field is measured by the spatial gradient (spatial rate of change) of the electric field. A fundamental operating principle of the dielectrophoresis effect is that the force (or torque) in air or other surrounding media generated at a point and space in time always points (or seeks to point) in the same direction, mainly toward the maximum gradient (non-uniformity) of the local electric field, independent of sign (+ or -) and time variations (DC or AC) of electrical fields (voltages) and of the surrounding medium dielectric properties.
The dielectrophoresis force magnitude depends distinctively nonlinearly upon the dielectric polarizibility of the surrounding medium, the dielectric polarizibility of initially neutral matter and nonlinearly upon the neutral matter's geometry. This dependance is via the Clausius-Mossotti function, well-known from polarizibility studies in solid state physics. The dielectrophoresis force depends nonlinearly upon the local applied electric field produced by the target. The dielectrophoresis force depends upon the spatial gradient of the square (second power) of the target's local electric field distribution at a point in space and time where a detector is located. The spatial gradient of the square of the local electric field is measured by the dielectrophoresis force produced by the induced polarization charge on the detector. This constant-direction-seeking force is highly variable in magnitude both as a function of angular position (at fixed radial distance from the target) and as a function of the radial position (at a fixed angular position) and as a function of the "effective" medium polarizibility. The force's detection signature is a unique pattern of the target's spatial gradient of the local electric field squared, with the detector always pointing (seeking to point) out the direction of the local maximum of the gradient pattern. All experimental results and equations of dielectrophoresis are consistent with the fundamental electromagnetic laws (Maxwell's equations).
There are five known modes of dielectric polarization. These include: electronic polarization, where electron distribution about the atom nuclei is slightly distorted due to the imposed external electric field; atomic polarization, where the atoms' distributions within initially neutral matter are slightly distorted due to the imposed external electric field; nomadic polarization, where in very specific polymers, etc. highly delocalized electron or proton distributions are highly distorted over several molecular repeat units due to the imposed external electric field; rotational polarization (dipolar and orientational), where permanent dipoles (H.sub.2 O, NO, HF) and orientable pendant polar groups (--OH, --Cl, --CN,--NO.sub.2) hung flexibly on molecules in material are rotationally aligned toward the external electric field with characteristic time constants; and interfacial (space charge) polarization, where inhomogeneous dielectric interfaces accumulate charge carriers due to differing small electrical conductivities. With the interfacial polarization, the resulting space charge accumulated to neutralize the interface charges distorts the external electric field with characteristic time constants.
The first three modes of dielectric polarization, electronic, atomic and nomadic, are molecular in distance scale and occur "instantaneously" as soon as the external electric field is imposed and contribute to the dielectric constant of the material at very high frequencies (infrared and optical). The last two polarization modes, rotational and interfacial, are molecular and macroscopic in distance scale and appear dynamically over time with characteristic time constants to help increase the high frequency dielectric constant as it evolves in time toward the dielectric constant at zero frequency. These characteristic material time constants control the dielectric and mechanical response of a material.
The modes of polarization and their dynamics in contributing to the time evolution of dielectric constants are discussed in various publications, such as H. A. Pohl, Dielectrophoresis, Cambridge University Press (1978); R. Schiller Electrons in Dielectric Media, C. Ferradini, J. Gerin (eds.), CRC Press (1991), and R. Schiller, Macroscopic Friction and Dielectric Relaxation, IEEE Transactions on Electrical Insulation, 24, 199 (1989), the well-known teachings of which are hereby incorporated by reference.
If an external electrical field E.sub.0 is applied to a dielectric material, the force (F) has a volume density (f=F/v) that includes forces on free charges, bound pairs of charges acting as polarizable dipoles, interactions between the dipoles and dimension changes due to the electric field (E) inside the dielectric material. The general volume force density can be defined in accordance with the following relation: ##EQU1## where .epsilon.=dielectric permitivity of the material (which equals K.epsilon..sub.0 where K is the dielectric constant of the material and .epsilon..sub.0 is the dielectric permitivity of free space), .alpha.=polarizibility of the dielectric material, .gradient.=spatial gradient vector mathematical operator, .differential..epsilon./.differential..rho..sub.mass =partial differentiation mathematical operator, .rho..sub.charge =volume density of free charges (carriers), and .rho..sub.mass =volume mass density of the dielectric material.
In the vast majority of dielectrics, .rho..sub.charge =0 so there is no electrostatic force to be considered. Similarly, with the exception of piezoelectric materials, .differential..epsilon..differential..rho..sub.mass =0 (i.e., there is no density variation in the dielectric constant), and no electrostriction force has to be considered. The two dielectrokinesis forces, the dipole-E.sub.0 field force and the dielectrophoresis force, thus remain to be considered.
The first dielectrokinetic force equals zero if the vector gradient of the dielectric permitivity .epsilon.=K.epsilon..sub.0 is zero (i.e., there is no spatial variation in the effective dielectric constant). If there is some spatial variation in the dielectric constant, then a relatively large force occurs, since the second term in Equation (1) is multiplied by the electric field squared. A simple example of the first dielectrokinesis force is where a warm liquid (having a lower dielectric constant than cold liquid, and therefore a non-zero spatial gradient) is set in motion toward the lower electric field regions. In a complex dielectric body, if .gradient..epsilon.=0 then all parts of a body are spatially matched dielectrically. The dielectric permitivity .epsilon. is a complex material parameter, in particular, for "pure" polymers, as well as "plastics" which are often mixtures or blends of polymers with additives to overcome chemical processing challenges and end-use product functional limitations. See D. W. van Krevelen, Properties of Polymers and Correlation to Chemical Structure, Elsevier Press (1976), the teachings of which are hereby incorporated by reference.
The third term in Equation (1), the dielectrophoresis force, enunciated by H. Pohl, involves the spatial gradient of the electric field squared. Hence, this second dielectrokinesis force is smaller than the dipole-dipole dielectrokinesis force.
Therefore, the net force density can be expressed as: EQU f=F/v=-1/2(E.sub.0 E) .gradient..epsilon.+1/2 .alpha..gradient.(E.sub.0 E) EQU f=dipole-E.sub.0 force+dielectrophoresis force (2)
The electrical energy density (U) stored in a dielectric body can be expressed as: EQU F=-.gradient.U (3)
whereby the energy (U) is the volume integral of the two electrokinesis forces involved.
Therefore, one of two situations, can occur: (1) .gradient..epsilon. does not equal zero, and the first dielectrokinesis force in Equation (2) is dominant (i.e., the various parts of the complex dielectric body are not dielectrically spatially matched) and the total energy of the system is large with large variations. This situation denotes "no match detected;" or (2) .gradient..epsilon. equals zero, and the dielectrophoresis (Pohl) force in Equation (2) is dominant (i.e., the various parts of the complex dielectric body are dielectrically spatially matched) and the total energy of the system is small with small variations. This situation denotes "match detected."
In situation 2 (match detected), force density (f) is expressed as: EQU f=F/v=1/2 G .gradient..vertline.K.sub.1 .epsilon..sub.0 E.sub.0 E.sub.0 .vertline. (4) EQU f=F/v=1/2 G .gradient..vertline.2U.sub.0 .vertline. (5)
where E=G E.sub.0 converts the electrical field in dielectric (E) to external field (E.sub.0), G=3 (K.sub.2 -K.sub.1)/(K.sub.2 +2K.sub.1) for spherically shaped dielectric objects and G=2 (K.sub.2 -K.sub.1)/(K.sub.2 +K.sub.1) for cylindrically shaped dielectric objects (with K.sub.2 being the dielectric constant of the material in the sphere or cylinder that is dielectrically spatially matched to a reference sample, and with K.sub.1 being the dielectric constant of the surrounding fluid (gas or liquid)), and U.sub.0 the electrical energy density "stored" in the external electric field E.sub.0.
It would be advantageous utilizing the concepts noted above to enable the detection of polymers and plastics and other organic/inorganic materials irrespective of the presence of intervening vision obstructing structures, barriers or EMI signals.